The features systemd has and encapuslates fits very well of what a service manager should do.

Just starting a service at boottime doesn't help much and ignores a lot of stuff around.

What do i start when a user logs in? how many resources is the service allowed to use? How do i control that service and read its logs?

If i already start a service, i can also start a service based on other events like bus events (which are needed potentially anyway for services to communicate) or by time.

And all of this needs to shutdown properly when finished.